Wigner's friend
Encyclopedia
Wigner's friend is a thought experiment
Thought experiment
A thought experiment or Gedankenexperiment considers some hypothesis, theory, or principle for the purpose of thinking through its consequences...

 proposed by the physicist
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

 Eugene Wigner; it is an extension of the Schrödinger's cat
Schrödinger's cat
Schrödinger's cat is a thought experiment, usually described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects. The scenario presents a cat that might be...

 experiment designed as a point of departure for discussing the Quantum mind/body problem
Quantum mind/body problem
The quantum mind–body problem refers to the philosophical discussions of the mind–body problem in the context of quantum mechanics. Since quantum mechanics involves quantum superpositions, which are not perceived by observers, some interpretations of quantum mechanics place conscious observers in a...

.

The thought experiment

The Wigner's Friend thought experiment
Thought experiment
A thought experiment or Gedankenexperiment considers some hypothesis, theory, or principle for the purpose of thinking through its consequences...

 posits a friend of Wigner who performs the Schrödinger's cat
Schrödinger's cat
Schrödinger's cat is a thought experiment, usually described as a paradox, devised by Austrian physicist Erwin Schrödinger in 1935. It illustrates what he saw as the problem of the Copenhagen interpretation of quantum mechanics applied to everyday objects. The scenario presents a cat that might be...

 experiment after Wigner leaves the laboratory. Only when he returns does Wigner learn the result of the experiment from his friend, that is, whether the cat is alive or dead. The question is raised: was the state of the system a superposition
Quantum superposition
Quantum superposition is a fundamental principle of quantum mechanics. It holds that a physical system exists in all its particular, theoretically possible states simultaneously; but, when measured, it gives a result corresponding to only one of the possible configurations.Mathematically, it...

 of "dead cat/sad friend" and "live cat/happy friend," only determined when Wigner learned the result of the experiment, or was it determined at some previous point?

Consciousness and measurement

Wigner designed the experiment to illustrate his belief that consciousness
Consciousness
Consciousness is a term that refers to the relationship between the mind and the world with which it interacts. It has been defined as: subjectivity, awareness, the ability to experience or to feel, wakefulness, having a sense of selfhood, and the executive control system of the mind...

 is necessary to the quantum mechanical measurement process. If a material device is substituted for the conscious friend, the linearity of the wave function implies that the state of the system is in a linear sum of possible states. It is simply a larger indeterminate system.

However, a conscious observer (according to his reasoning) must be in either one state or the other, hence conscious observations are different, hence consciousness is not material. Wigner discusses this scenario in "Remarks on the mind-body question", one in his collection of essays, Symmetries and Reflections, 1967. The idea has become known as the consciousness causes collapse interpretation.

Consciousness and Superposition

A counterargument is that the superimposition of two conscious states is not paradoxical — just as there is no interaction between the multiple quantum states of a particle, so the superimposed consciousnesses need not be aware of each other.

Just as in any Many-worlds interpretation
Many-worlds interpretation
The many-worlds interpretation is an interpretation of quantum mechanics that asserts the objective reality of the universal wavefunction, but denies the actuality of wavefunction collapse. Many-worlds implies that all possible alternative histories and futures are real, each representing an...

, a bad world-count can lead to different probabilities.

Wigner's friend in Many Worlds

The Many worlds interpretation avoids the need to postulate that consciousness causes collapse — indeed, that collapse occurs at all. Unconscious systems split (decohere) when there is an irreversible difference between their state in the world where the cat survived or will survive and their Counterpart
Counterpart theory
In philosophy, specifically in the area of modal metaphysics, counterpart theory is an alternative to standard possible-worlds semantics for interpreting quantified modal logic. Counterpart theory still presupposes possible worlds, but differs in certain important respects from the Kripkean view...

 in the other case. In the example below this happens with the cyanide device and the telephone. Similarly, conscious systems split when there is an irreversible difference between their state in the world where the cat survived (or will survive) and their Counterpart
Counterpart theory
In philosophy, specifically in the area of modal metaphysics, counterpart theory is an alternative to standard possible-worlds semantics for interpreting quantified modal logic. Counterpart theory still presupposes possible worlds, but differs in certain important respects from the Kripkean view...

 in the other case. In the example below, this happens with the cat, Wigner's friend and Eugene Wigner.

According to “Many worlds”, when Wigner's friend (the investigator) finds out the result of the Schrödinger's cat experiment, the part of the world where the friend is 'decoheres irreversibly' or splits. http://www.hedweb.com/manworld.htm#decoherence In one split world the friend observes a live cat. In the other the friend observes a dead cat.

So, it is maintained that Eugene Wigner splits when there is an irreversible difference between Wigner in the world where the cat survived and Wigner’s counterpart in the world where the cat died. In the original thought experiment Wigner postulated that he would find out when he returned to the laboratory. There can be other cases.

For example, perhaps in the world where the cat survived Wigner's friend may telephone at once with the good news. In the world where the cat died Wigner may find out later. In that case when Wigner's friend makes the telephone call in one world Eugene Wigner splits into two. One counterpart knows the result. The other counterpart does not know.

Objective Collapse Theories

According to objective collapse theories, wave function collapse occurs when a superposed systems reaches a certain objective threshold of size, complexity etc. Objective collapse proponents
would expect a system as macroscopic as a cat to have collapsed before the box was open, so the question of observation-of-observers does not arise for them.

Sources

Wigner's original remarks about his friend appeared in his article "Remarks on the Mind-Body Question", published in the book "The Scientist Speculates", edited by I. J. Good. The article is reprinted in Wigner's own book Symmetries and Reflections.
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
x
OK